VNAV path deviation indication during deceleration?

[Hardy Heinlin]

Good morning,

regarding the deviation indication, it could be that PSX's FMC provides more luxury than the real FMC: In PSX the sizes and locations of the deceleration "knees" are calculated in advance and inserted in the predicted overall idle descent path, so that the deviation indication will stay pretty well centered when the aircraft nose goes up for a -500 fpm sinkrate for airspeed deceleration.

I could imagine, when the deceleration starts, the real FMC blanks the indication and recalculates the path for the new, lower command speed, then it re-appears and indicates the aircraft is intercepting the new path from below.

Does anybody know more about it?


Thanks,

|-|ardy

[emerydc8]

That's a good question. Whatever it does, the airplane usually stays right on path. Whether that path is artificially modified to a shallower descent gradient during the deceleration, I don't know. I'm pretty sure it doesn't show a new path during deceleration where the aircraft would re-intercept from below. Maybe Peter can chime in.

Jon

[Hardy Heinlin]

If you say it stays right on path during deceleration, then it's clear that it does calculate the knee. (Which is also the case in PSX.)


|-|ardy

[Hardy Heinlin]

Avi, your last "jet stream" test inspired me to rewrite the pitch control algorithm for the HOLD | VNAV PTH mode :-)

I'm testing and tuning it right now; it's actually simpler than the previous algorithm, but the point is: It tries to keep the path deviation at zero and accepts any airspeed additive, instead of trying to keep both airspeed additive and path deviation at a balanced minimum.

So the elevator job is no longer a combination of path and airspeed protection, but just path protection. And the airspeed protection is provided solely by the mode change logic (A/T SPD when below, VNAV SPD when above).

The mode logic itself (and its trigger values) remains as is.


Regards,

|-|ardy

[emerydc8]

And the airspeed protection is provided solely by the mode change logic (A/T SPD when below, VNAV SPD when above).

Just to clarify this, do you agree that with an unforecast tailwind it should stay in VNAV PTH (exactly on path) and accelerate up to VMO-11 (when above 1st speed constraint) or up to 15 knots above target (when below 1st speed constraint and subject to 5 knots above transition speed or 5 knots below flap placard speed)?

Then, if the increased speed won't allow you to stay exactly on path, it will allow you to rise up to 150" above path before speed reversion occurs. Only then will it go to IDLE/HOLD||VNAV SPD.

Jon

[Hardy Heinlin]

Just to clarify this, do you agree that with an unforecast tailwind it should stay in VNAV PTH (exactly on path) and accelerate up to VMO-11 (when above 1st speed constraint) or up to 15 knots above target (when below 1st speed constraint and subject to 5 knots above transition speed or 5 knots below flap placard speed)?

Yes. But it's VMO-16 when in ECON.

Then, if the increased speed won't allow you to stay exactly on path, it will allow you to rise up to 150" above path before speed reversion occurs. Only then will it go to IDLE/HOLD||VNAV SPD.

Most likely 150. But it certainly includes closure rates and other dampening variables.


I'm not going to change the logic. Just the PTH pitch control.


|-|ardy

[emerydc8]

Yes. But it's VMO-16 when in ECON.

Thanks, Hardy. You're right. It will go up to VMO-11 (354 knots) only if it is a pilot-entered speed, which no one ever enters. So, VMO-16 it is.

I'm not going to change the logic. Just the PTH pitch control.

I think you are right in making the path deviation zero. Also, the situation where it would have to rise up to 150' above the path after reaching VMO-16, or target +15 below the first speed constraint, would likely be rare enough anyway; so that' feature would not normally be observed, unless you went out of your way to add a large tailwind during descent, just to see what it would do.

Jon

[Hardy Heinlin]

By the way, it's pretty easy to reach VMO-16 even without unexpected tailwinds if you have a high cost index that puts you near VMO, especially at very high altitudes where your Mach number hasn't much room between the red "coffin corners".


|-|ardy

[emerydc8]

True. Although with the low cost index my airline uses, the only time you'd be cruising anywhere near VMO would be if you are higher than you should be or you manually inserted a higer speed into the VNAV page 2/3 or speed intervened.

[Will]

What CI does your airline usually use?

[emerydc8]

030 - 040.

[Will]

So for 030-040, do that usually round it off somehow, like 030, 035, and 040? Or do they give you the full range of possible integers: 030, 031, 032, 033, etc.?

[andrej]

030 - 040.

You guys are flying slow. But hey...at least you are little longer in the cockpit.

[emerydc8]

So for 030-040, do that usually round it off somehow, like 030, 035, and 040? Or do they give you the full range of possible integers: 030, 031, 032, 033, etc.?

Whatever is on the flight plan is what we use.

Jon